hr) The most common unit for specific impulse is the second, both in SI contexts as well as where imperial or customary units are used. If mass (kilogram or slug) is used as the unit of propellant, then specific impulse has units of velocity. To calculate the specific impulse, we first need to calculate the exhaust velocity. Previously we used the steady flow energy equation to relate the exhaust velocity of a rocket motor, Figure 14.1, to the conditions in the combustion chamber and the exit pressure. Note that effective exhaust velocity in the metric system is also the specific impulse value in the metric system for optimum expansion ratio. 14. The word "specific" just means "divided by weight". hr) The most common unit for specific impulse is the second, both in SI contexts as well as where imperial or customary units are used. A thruster with a specific impulse of 408 seconds used in orbit around the Pluto will have an effective exhaust velocity of about 4000 m/s. A ground based propulsion laser on Earth beams light onto a focusing mirror attached to the spacecraft. By definition, it is the impulse delivered per unit of propellant consumed, and is dimensionally equivalent to the thrust generated per unit propellant flow rate. Assuming the equivalent velocity remains constant with time, we can integrate the equation to get: I = m * Veq . The Tsiolkovsky rocket equation, classical rocket equation, or ideal rocket equation is a mathematical equation that describes the motion of vehicles that follow the basic principle of a rocket: a device that can apply acceleration to itself using thrust by expelling part of its mass with high velocity can thereby move due to the conservation of momentum. Specific impulse, as the name suggests is $ Specific Impulse = \frac{Impulse}{Mass} = \frac {Force \cdot time}{Mass}$ It has the units of velocity, i.e. It's a common measure of the "mass efficiency" of a rocket engine. When a rocket is reported to have a specific impulse of 1000 m/sec, it means that the velocity of gases exiting the nozzle is 1000 m/sec relative to the nozzle. We can divide this equation by the weight of the propellants to define the specific impulse. That specific impulse is typically expressed in seconds in the US is essentially a reflection on the goofy system of measurements used in the US -- as Astronuc already said, thrust per weight flowrate. where m is the total mass of the propellant. You can use the radio buttons above to convert between specific impulse and exhaust velocity. Simply put, specific impulse is a measure of how much push (thrust) accumulates as you burn the fuel. As the final velocity becomes multiples of the exhaust velocity, fuel : payload increases exponentially . This is simply the speed at which burning propellants leave the rocket engine, and for a typical liquid oxygen and liquid hydrogen rocket engine, this is about [math]4,400 m/s[/math] in a vacuum.
Specific Impulse is exhaust velocity divided by g, the acceleration due to gravity on Earth's surface (9.80665 m/s^2). $ m s^{-1} $ or $ ft s^{-1} $ and so according to one's education its numerical value varies. 1 Thrust and Specific Impulse for Rockets Previously we used the steady flow energy equation to relate the exhaust velocity of a rocket motor, Figure 14.1, to the conditions in the combustion chamber and the exit pressure. 14.
The next step is to divide … Specific impulse (usually abbreviated I sp) is a measure of the efficiency of rocket and jet engines. Specific impulse (usually abbreviated I sp) is a measure of the efficiency of rocket and jet engines. It represents the force with respect to the amount of propellant used per unit time. Since the real exhaust velocity is exceeding complex to calculate, we will be using some simplifying assumptions to make a simpler equation. Let’s start with exhaust velocity. 1 Thrust and Specific Impulse for Rockets . The Solution to High Thrust, High Specific Impulse Spaceflight A laser powered plasma rocket is a straightforward solution. Specific impulse is a measure of efficiency for rocket engines. More specifically, you could say that it is the change in momentum per unit of mass for rocket fuels.